From The Link Reaction To The Krebs Cycle
- In the link reaction (covered in C1.2.11), pyruvate is oxidized and decarboxylated to produce acetyl-CoA.
- Acetyl-CoA carries a 2-carbon acetyl group that still contains significant chemical energy.
- The Krebs cycle is where this acetyl group is completely broken down to extract that energy.
Definition
The Krebs cycle
The Krebs cycle (also known as the citric acid cycle) occurs in the mitochondrial matrix and is a key part of aerobic respiration.
Step-by-Step: How the Krebs Cycle Works
Step 1: Acetyl Group Enters the Cycle
- The acetyl group (2C) from acetyl-CoA is transferred to a 4-carbon molecule called oxaloacetate.
- This forms a 6-carbon molecule called citrate.
- Coenzyme A is released and recycled back to the link reaction.
2C (acetyl) + 4C (oxaloacetate) = 6C (citrate)
Exam techniqueYou only need to remember two molecules for the exam: citrate (6C) and oxaloacetate (4C).
Step 2: Removing Hydrogens (Oxidation/Dehydrogenation)
- As citrate moves through the cycle, hydrogen atoms are systematically removed.
- Each hydrogen atom consists of a proton (H⁺) and an electron (e⁻).
- These hydrogens are transferred to carrier molecules:
- NAD⁺ accepts hydrogens and becomes NADH (happens 3 times).
- FAD accepts hydrogens and becomes FADH₂ (happens 1 time).
Total oxidations per cycle: 4 (3 produce NADH, 1 produces FADH₂)
Step 3: Removing Carbons (Decarboxylation)
- The cycle removes carbon atoms as carbon dioxide (CO₂).
- This happens twice during the cycle.
- Each decarboxylation releases one carbon as waste.
The carbon count drops: 6C (citrate) → loses 1C (first decarboxylation) → 5C intermediate → loses 1C (second decarboxylation) → 4C
Note- The two carbons from the original acetyl group are released as waste.
- They leave the mitochondria, enter the bloodstream, and are exhaled.
Step 4: Direct ATP Production
- During one step of the cycle, 1 ATP (or GTP) is produced directly.
- A phosphate group is transferred from an intermediate to ADP, forming ATP.
- This is called substrate-level phosphorylation.
Step 5: Oxaloacetate is Regenerated
- After four oxidations and two decarboxylations, the molecule is back to 4 carbons.
- This is oxaloacetate (4C), the same molecule from Step 1.
- Oxaloacetate can now accept another acetyl group and restart the cycle.
- The cycle is self-renewing.
- Oxaloacetate isn't consumed but regenerated, allowing the cycle to continue indefinitely as long as acetyl-CoA is available.
Summary of Products Per Acetyl Group
- Each acetyl group that goes through the Krebs cycle produces:
- 3 NADH (from Step 2: oxidations)
- 1 FADH₂ (from Step 2: oxidations)
- 1 ATP (from Step 4: substrate-level phosphorylation)
- 2 CO₂ (from Step 3: decarboxylations—waste)
- Remember that one glucose produces 2 acetyl-CoA in the link reaction, so the cycle runs twice per glucose.
- Multiply all products by 2 for totals per glucose.
- What enters the Krebs cycle from the link reaction?
- What is formed when the acetyl group combines with oxaloacetate?
- How many oxidations occur per turn of the Krebs cycle?
- How many decarboxylations occur per turn of the Krebs cycle?
- What happens to the 2 carbons from the acetyl group?
- Why can the Krebs cycle keep running indefinitely?
- What is the main product of the Krebs cycle that fuels ATP production?
